Block removal and step backwearing as erosion processes on rock shore platforms: a preliminary case study of the chalk shore platforms of south‐east England

Abstract

Shore platforms frequently exhibit steps or risers facing seaward, landwards or obliquely across-shore. A combination of soft copy photogrammetry, ortho-rectification, geo referencing and field measurement of step height are linked in a GIS environment to measure step retreat on chalk shore platforms at sample sites in the south of England over two periods, 1973-2001, 2001-2007. The methods used allow for the identification, delineation and measurement of historic change at high spatial resolution. The results suggest that while erosion of chalk shore platforms by step backwearing is highly variable, it appears to be of similar magnitude to surface downwearing of the same platforms measured by micro-erosion meters (MEMs) and laser scanning, in a range equivalent to 0·0006 -0·0050 m y -1 of surface downwearing. This equates to annual chalk volume loss from the platforms, by the two erosion processes combined, of between 0·0012 m 3 m -2 and 0·0100 m 3 m -2. Results from the more recent years' data suggests that step retreat has variability in both space and time which does not relate solely to climatic variability. The results must be viewed with caution until much larger numbers of measurements have been made of both downwearing and step erosion at higher spatial and temporal resolution.

Controls on the distribution of cosmogenic <sup>10</sup>Be across shore platforms

“…Several coastal platforms have been observed to erode due to quarrying and block removal (e.g. Dornbusch and Robinson, 2011;Naylor et al, 2016). In order to explore the potential implication of these erosion processes for the accumulation of 10 Be in a shore platform, we also evolved a series of stepped platforms by steady-state retreat.…”

“…Where available, information on the rate at which steps migrate and blocks are removed will help to inform sampling and interpretation (e.g. Dornbusch and Robinson, 2011;Naylor et al, 2016). Measuring 10 Be concentrations on shore platforms where block removal processes are dominant may allow the rate of step migration to be determined in the presence of large bedrock steps (Fig.…”

“…weathering and abrasion) or episodic (e.g. quarrying) processes (Dornbusch and Robinson, 2011;Naylor et al, 2016), and the relative influence of these processes on the distribution of 10 Be has yet to be explored. Similarly, the assumption that shore platforms evolve in morphological steady state (i.e.…”

Recent acceleration in coastal cliff retreat rates on the south coast of Great Britain

“…A variety of cliff failure mechanisms have been observed, including vertical collapses, wedge collapses, rockfalls, rotational failures, and toppling (17); all of these processes can result in several meters of clifftop retreat in a single event. Erosion of platforms seems to occur through a combination of vertical downwearing due to frost action, mechanical and biological abrasion (22), and subhorizontal step retreat (23).…”

Late Holocene seacliff retreat recorded by 10Be profiles across a coastal platform: Theory and example from the English Channel

“…3 w=v tan α where w is the downwearing rate and v is horizontal cliff retreat rate (Dornbusch and Robinson, 2011). The basic model takes water shielding into account, but for simplicity ignores factors like glacial-age cliff smoothing and diffusion, chalk weathering and vegetation shielding (see expected effects of these factors in the discussion section).…”